1. Field of the Invention
This invention generally relates to an apparatus for reproducing a digital signal. This invention particularly relates to an apparatus for processing a signal read out from a recording medium such as an optical disc to reproduce a digital signal therefrom.
2. Description of the Related Art
Some recording mediums such as optical discs store signals representative of digital information or binary information. In general, a signal reproduced from such a recording medium has an analog waveform reflecting a binary digital signal. The reproduced analog-waveform signal is compared with a slice level, being converted into a reproduced binary digital signal indicating a stream of bits each being “0” or “1”.
Regarding rewritable optical discs, the recording sensitivity varies from disc to disc. Optical discs are accessed by optical heads using semiconductor lasers. The variation in recording sensitivity and the ageing of semiconductor lasers cause variations in shapes of signals recorded on optical discs. Due to the variations in the recorded signals, analog-waveform signals reproduced from the optical discs tend to change in DC components or to be asymmetric between upper waveform portions and lower waveform portions. In order to compensate for these undesirable changes of the reproduced signals, slice levels for reproducing binary digital signals are controlled.
In a first known control procedure, the DC components of a reproduced analog-waveform signal is detected, and a slice level is controlled in response to the detected DC components. In a second known control procedure, the error of the duty cycle of a binary digital signal reproduced from an analog-waveform signal is detected, and a slice level is controlled in response to the detected error. In a third known control procedure, the DC components of a reproduced analog-waveform signal is detected, and the DC level of the reproduced analog-waveform signal is controlled in response to the detected DC components to implement relative control of a slice level. In a fourth known control procedure, the error of the duty cycle of a binary digital signal reproduced from an analog-waveform signal is detected, and the DC level of the analog-waveform signal is controlled in response to the detected error to implement relative control of a slice level.
According to the DVD (digital versatile disc) standards, the error of the duty cycle of a binary digital signal reproduced from an analog-waveform signal is detected, and a slice level is controlled in response to the detected error.
The slice-level control responsive to the duty-cycle error or the DC signal components tends to be useless in the case where a binary digital signal represented by a reproduced analog-waveform signal has an appreciable difference in number between “0” and “1”.
Known DVD recording sides suppress low-frequency components of modulation-resultant recorded signals by using a DSV (digital sum value or digital sum variation) control procedure, a substitute encoding table, a sync pattern, or connection bits. The suppression of the low-frequency signal components is good for control of slice levels in DVD reproducing sides.